Hot-melt polyesterimide-polyisocyanate electrical coating compositions

ABSTRACT

Hot-melt electrical coating compositions comprise high solids content polyesterimides prepared to an acid number of less than 6-7 in the presence of a monoether or monoester of a glycol or polyglycol and reacted hot with a blocked polyisocyanate. Such virtually solventless compositions provide insulation coatings on electrical conductors which have superior flexibility, especially after aging.

This is a division, of application Ser. No. 154,838 filed May 30, 1980,now U.S. Pat. No. 4,296,229.

This invention relates to high solids contentpolyesterimide-polyisocyanate coating compositions and to electricalconductors hot-melt coated therewith. More particularly, it is concernedwith virtually solvent-free polyesterimides prepared in the presence ofa small amount of a monoether or monoester of a glycol, or polyglycol,and thereafter heat reacted with a blocked polyisocyanate.

BACKGROUND OF THE INVENTION

Schmidt et al., U.S. Pat. No. 3,697,471, disclose a family of esterimide resins made by reacting together at least one polybasic acid or afunctional derivative thereof, and at least one polyhydric alcohol orfunctional derivative thereof, at least one of the reactants having atleast one five-membered imide ring between the functional groups of themolecule. It further discloses that the reactants can be heated in acommercial cresol mixture, then further diluted in a mixture of naphthaand cresol and used as an enamel for coating copper wire to produce ahard, thermally resistant insulation therefor. Meyer et al., U.S. Pat.No. 3,426,098, describe ester imide resins in which all or part of thepolyhydric alcohol comprises tris(2-hydroxyethyl) isocyanurate.

Keating, U.S. Pat. Nos. 4,119,605, 4,119,608; and 4,119,758,incorporated herein by reference, discloses low molecular weightpolyester-imide resins which are soluble in non-phenolic solvents byincorporating a monoether of a diethylene glycol or a triethylene glycolinto the resin. Keating states that the use of the monoether of a glycolas a reactant serves to control the molecular weight and solubility ofthe polymer and permits the manufacture of wire enamels which containrelatively non-toxic solvents in place of phenols and also permits theattainment of enamels having economical high solids contents. Boldebuck,Banucci and Byrne in commonly-assigned copending application Ser. No.867,939, filed Jan. 9, 1978, disclose a way of preparing virtuallysolvent-free polyesterimides, suitable for hot melt coating, by addingto the reaction mixture a monofunctional alcohol-reactant, such as decylalcohol or a glycol ether.

Sheffer et al., U.S. Pat. No. 2,982,754, and Meyer et al., U.S. Pat. No.3,211,585, describe cresylic acid-solubilized polyester-polyisocyanatesfor use as wire enamels.

Zamek, in commonly-assigned copending application Ser. No. 100,212,filed Dec. 4, 1979, describes in Example 5 the production of a partiallycompletely reacted polyesterimide (acid number 6-7) and its reduction bya glycol ether solvent prior to reaction in solution with a blockedpolyisocyanate to produce a coating composition which is an extremelyviscous liquid at room temperature and has a solids content of 67.72%.It is stated to be adapted to be hot applied to copper wire at 80° C.The foregoing patents and applications are incorporated herein byreference.

It has now been discovered that virtually solventless reaction productsof polyesterimides and blocked isocyanates can be made for use as wireenamels applied from the melt. As mentioned above, typically, wires arecoated with enamels that are solutions of from 20 to 50% solids incresylic acid and non-cresylic acid solvent systems. Application ofthose enamels require that the solvents be removed before cure andsubsequently burned or released into the atmosphere. This procedureinvolves substantial use of energy and has environmental shortcomings.The present invention avoids these problems.

The hot melt polyesterimide component is produced herein bypolymerization of the reactants at extremely high solids content (>75%)and direct isolation of the reaction product with blocked polyisocyanateat the end of the reaction. An important aspect of the invention is theproduction of a resin with the appropriate viscosity at the various hotmelt applicator temperatures and the ability to add the necessary curingadditives, e.g., the blocked polyisocyanate, to the polyester imidecomponent at high temperature without the aid of a solvent. In thepresent invention, the use of a minimum amount of a glycol ether orester, such as the monomethyl ether of diethylene glycol or themono-n-butyl ether of diethylene glycol as a reaction mixture componentaids in processing, e.g., the removal of volatile by-products andtemperature control, and such monofunctional compounds function as achain stopper limiting molecular weight and thus high temperatureviscosity. The blocked polyisocyanates are introduced at higher thannormally expected temperatures, even at 180° C. and, surprisingly, nopremature cure is observed. In any case, it is preferred to employ aconventional transesterification catalyst, such as a titanate, and thiscan be added all in the beginning or some at the beginning and thebalance at the end.

DESCRIPTION OF THE INVENTION

According to the present invention, there are provided compositions forhot-melt electrical wire coating comprising:

a high solids content resinous polyesterimidepolyisocyanate reactionproduct obtained by heating ingredients comprising:

(a) an aromatic diamine;

(b) an aromatic carboxylic anhydride containing at least one additionalcarboxylic group;

(c) terephthalic acid or a reactive derivative thereof;

(d) a polyhydric alcohol having at least three hydroxyl groups;

(e) an alkylene glycol; and a small amount of

(f)

(i) an alkylene glycol monoether or monoester;

(ii) a polyalkylene glycol monoether or monoester; or

(iii) a mixture of (i) and (ii),

until an acid number of lower than 6 to 7 is attained, and adding to thehot reaction product a blocked polyisocyanate in an amount of 1 to 40parts by weight per 100 parts by weight of the polyesterimide andblocked polyisocyanate.

Among the preferred features of the present invention are electricalcoating compositions as defined above in which the solids content is atleast 75 parts by weight; those in which heating is carried out at atemperature from about 188° to about 220° C., and the blockedpolyisocyanate is added at a temperature of from about 120° C. to about180° C.; and those which also include an alkyl titanate.

Also contemplated by the present invention are electrical conductorsprovided with a continuous coating of the new wire enamels, as a solecoat, or as an undercoat, or as an overcoat, and cured at elevatedtemperatures.

With respect to components (a)-(e) inclusive, these are conventional andwell known to those skilled in this art by reason of the teachings, forexample, in the above-mentioned U.S. Pat. No. 3,697,471 and 3,426,098.

By way of illustration, aromatic diamine component (a) can comprisebenzidine, methylene dianiline, oxydianiline, diaminodiphenyl ketone,-sulfone, -sulfoxide, phenylene diamine, tolylene diamine, xylenediamine, and the like. Preferably, component (a) will compriseoxydianiline or methylenedianiline, and, especially preferably,methylenedianiline.

Illustratively, the aromatic carboxylic anhydride containing at leastone additional carboxylic group component (b) can comprise pyromelliticanhydride, trimellitic anhydride, naphthalene tetracarboxylicdianhydride, benzophenone-2,3,2',3'-tetracarboxylic dianhydride, and thelike. The preferred components (b) are pyromellitic anhydride ortrimellitic anhydride and especially trimellitic anhydride.

Typically, terephthalic acid or a di(lower) alkyl ester (C₁ -C₆) orother reactive derivative, e.g., amide, acyl halide, etc., will be usedas component (c). A minor amount of the terephthalic acid can bereplaced with another dicarboxylic acid or derivative, e.g., isophthalicacid, benzophenone dicarboxylic acid, adipic acid, etc. Preferablycomponent (a) will comprise dimethyl terephthalate or terephthalic acid,and especially preferably, terephthalic acid.

As additional polyester forming ingredient (d) there will be empolyed apolyhydric alcohol having at least three hydroxyl groups. There can beused glycerine, pentaerythritol, 1,1,1-trimethylolpropane, sorbitol,mannitol, dipentaerythritol, tris(2-hydroxyethyl)isocyanurate (THEIC),and the like. Preferably as component (d) there will be used glycerineor tris(2-hydroxyethyl) isocyanurate, preferably the latter.

Illustratively, the alkylene glycol component (e) will comprise ethyleneglycol, 1,4-butanediol, trimethylene glycol, propylene glycol,1,5-pentanediol, 1,4-cyclohexane dimethanol and the like. Preferably,the alkylene glycol will be ethylene glycol.

The polar oxygenated reactant (f) having at least one active hydroxylgroup will be selected from (i) an alkylene glycol monoether ormonoester, such as ethylene glycol monomethyl ether, ethylene glycolmono-n-butyl ether, ethylene glycol monohexyl ether, ethylene glycolmonoacetate, ethylene glycol monohexanoate, propylene glycol monomethylether, propylene glycol monoacetate, 1,4-butylene glycol mono-n-butylether, 1,6-hexylene glycol monomethyl ether, 2-phenoxy-2-propanol, andthe like, and for (ii) a polyalkylene glycol monoether or monoester,such as diethylene glycol monoethyl ether, diethylene glycolmono-n-butyl ether, diethylene glycol monomethyl ether, diethyleneglycol monoacetate, diethylene glycol monophenyl ether, triethyleneglycol monoacetate, etc. Preferably, there will be used diethyleneglycol monomethyl or diethylene glycol mono-n-butyl ethers or diethyleneglycol monoethyl ethers.

The blocked polyisocyanates to be employed are known in the art, e.g.,from the above-mentioned U.S. Pat. Nos. 2,982,754 and 3,211,585. Theterm "blocked" means that the polyisocyanate has been reacted with agroup that will split off at the temperature employed with the polymericester imide. As polyisocyanate components there may be mentioned2,4-tolylene diisocyanates; 2,6-tolylene diisocyanates; cyclopentylenediisocyanate; m-phenylene diisocyanate; p-phenylene diisocyanate;ethylene diisocyanate; butylidene diisocyanate; 1,5-naphthalenediisocyanate; 1,6-hexamethylene diisocyanate; dianisidene diisocyanate;4,4'-diphenylether diisocyanate; 4,4',4"-triphenylmethane diisocyanate(Desmodur R); the cyclic trimer of 2,4-tolylene diisocyanate; mixturesof the cyclic trimers of 2,4-tolylene diisocyanate and 2,6-tolylenediisocyanate; the trimer of 4,4'-diphenyl 2,4,6-triisocyanato toluene,and the like. Typical compounds which can be used to block theisocyanate groupings, e.g., by forming carbonates therewith, aremonohydric phenols, such as phenol; meta-cresol; para-cresol; orthocresol and mixtures thereof; the xylenols, e.g., 2,6-dimethylphenol;4-ethyl phenol, 4-t-butylphenol; 2-butylphenol; 4-n-octylphenol;4-isooctylphenol, etc., monohydric alcohols, such as methyl alcohol;ethyl alcohol, n-propyl alcohol, and the like, acetoacetic ester,hydroxyalkylcarbamic acid aryl esters, e.g., hydroxyethylcarbamic acidphenyl ester, mercaptans, e.g., 2-mercaptobenzothiazole, methylmercaptans, and the like.

Others include the cyclic trimer of 2,4-tolylene diisocyanate having theisocyanate groups blocked with tertiary butyl alcohol or tertiary amylalcohol or dimethyl ethinyl carbinol or acetoacetic acid ester or phenolor cresylic acid or ε-caprolactam or 2-mercaptobenzothiazole, orsuccinimide or phthalimide or diphenyl amine or phenyl-β-naphthylamine,triphenyl methane triisocyanate having the isocyanate groups blockedwith phenol or mixed cresols or tertiary butyl alcohol or phthalimide,1,3,3-pentanetriisocyanate having the isocyanate groups blocked withm-cresol, etc.

As specific examples of such blocked isocyanates there may also bementioned Mondur S, wherein the isocyanate groups of the reactionproduct of 3 moles of mixed 2,4- and 2,6-tolylene diisocyanate withtrimethylol propane are blocked by esterification with phenol and MondurSH, wherein the mixed cyclic trimers of 2,4- and 2,6-tolylenediisocyanates have the three free isocyanates isocyanate groups blockedby esterification with m-cresol. Also preferred are alkanol-blockedpolyisocyanates and special mention is made of Mobay's KL-57005, in thisrespect.

Suitable alkyl titanates include tetraisopropyl titanate, tetramethyltitanate, tetrabutyl titanate, tetrahexyltitanate,bis(acetoacetonyl)diisopropyl titanate and the like. Amounts will rangefrom 0.2% to 1% present at the start of the reaction. Tetraisopropyltitanate is preferred.

In making the polyesterimide there should normally be an excess ofalcohol groups over carboxyl groups in accordance with conventionalpractice. The preferred ratios of ingredients, and of ester groups toimide groups, are entirely conventional, see the patents cited above,and the especially preferred ratios of ingredients will be exemplifiedin detail hereinafter. The polyesterimide can be prepared in two ways,both of which will yield enamels with improved properties. In one mannerof proceeding, all of the reactants are added to the vessel at thebeginning of the polymerization. The reaction is carried out in theusual manner, e.g., under by-product distillation conditions, e.g., at200° to 250° C., until the acid number drops below about 6-7 mg.KOH/pergram of sample, and preferably down to less than 1.0 then the reactionheating is discontinued, the blocked polyisocyanate is added to the hotmixture and it is maintained hot for the time needed to complete thereaction, e.g., 10-90 minutes, depending on the temperature. In anotherway, a two-stage reaction is conducted. First a hydroxyl rich polyesteris prepared from ingredients (c), (d) and (e), and at the completion ofthis reaction, then ingredients (a), (b) and (f) are added and thereaction carried further under by-product distillation conditions until,the acid number again falls below 6-7, e.g. to 1.0 or below. Heating isdiscontinued, then the blocked polyisocyanate is again added to the hotreaction mixture, as before. In either case the preferred titanate canbe added at the start of the reaction. The resin upon cooling to roomtemperature will either be a very viscous liquid or, more frequently, asolid.

Coatings prepared by using the compositions of the present inventionhave improved properties over glycol ether, or ester, based esterimidesthat have been prepared via reactions of the prior art. As has beenmentioned, and as will be shown hereinafter, the flexibility retentionis remarkably improved.

The wire enamels thus made are applied to an electrical conductor, e.g.,copper, aluminum, silver or stainless steel wire, in conventional hotmelt applications or by fluidized coating techniques, and the like.Illustratively, wire speeds of 15 to 65 feet/min. can be used with wiretower temperatures of 250° to 920° F. The build up of coating on thewire can be increased by repetitive passes through the resincomposition. The coatings produced from the present enamels haveexcellent smoothness, flex retention or flexibility, continuity, solventresistance, heat aging, dissipation factors, cut through resistance,heat shock, abrasion resistance and dielectric strength.

When used as an undercoat the enamels of this invention are applied tothe conductor as above-mentioned, and built up to the conventionalthickness, e.g., with multiple passes. Then a lesser wall of adifferent, overcoat enamel is applied. This can be, without limitation,a polyamide-imide, e.g., the heat reaction product of trimelliticanhydride and methylene dianiline diisocyanate, or an etherimide, apolyester, a nylon, an isocyanurated polyester, an isocyanuratedpolyester polyimide, and the like. When used as an overcoat, the enamelsof this invention are applied as a lesser wall over a conductorpreviously provided with an undercoat of a different enamel, such as apolyester or a polyester imide, etc. Suitable second-type enamels areshown, e.g., in Precopio et al., U.S. Pat. No. 2,936,296; Meyer et al.,U.S. Pat. No. 3,342,780; Meyer et al., U.S. Pat. No. 3,426,098; George,U.S. 3,428,486; and Olson et al., U.S. Pat. No. 3,493,413, all of whichare incorporated herein by reference to save unnecessarily detaileddescription.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples illustrate the present invention. They are notintended to limit the scope of the claims in any manner whatsoever.

EXAMPLE 1

A wire enamel is made by charging a suitably sized flask with thefollowing reactants:

    ______________________________________                                                             Gms.                                                     ______________________________________                                        Ethylene glycol        214.2                                                  Terephthalic acid      582.53                                                 Tris(2-hydroxyethyl)isocyanurate                                                                     820.71                                                 Tetraisopropyl titanate                                                                              22.2                                                   Diethylene glycol monomethyl ether                                                                   663.3                                                  Methylenedianiline     298.08                                                 Trimellitic anhydride  573.99                                                 ______________________________________                                    

The contents are slowly heated with the evolution of water to a maximumtemperature of 220° C. until no more distillate is evolved, and an acidnumber of 0.37 is achieved. The batch is cooled to 180° C. and there isadded 66.6 g. of a blocked polyisocyanate comprising mixed cyclictrimers of 2,4- and 2,6-tolylene diisocyanates having the three freeisocyanate groups blocked by esterification with m-cresol (MobayChemical Co., Mondur SH). The resin is stirred for an additional 30 min.while cooling and the hot resin is poured directly into cans. It is asolid at room temperature, has a solids content of 75% (after heating inan open dish at 200° C., for 1 hr.), and has a viscosity of 160 cps. at150° C. It is hot applied to copper wire at 160° C., to produce a coatedconductor with good continuity and acceptable surface.

EXAMPLE 2

A wire enamel is made by charging a suitably sized flask with Part I ofthe following reactants:

    ______________________________________                                                             Part Grams                                               ______________________________________                                        Ethylene glycol        I      321.3                                           Terephthalic acid      I      873.8                                           Tris(2-hydroxyethyl)isocyanurate                                                                     I      1231.1                                          Tetraisopropyl titanate                                                                              I      33.9                                            Diethylene glycol monomethyl ether                                                                   II     331.7                                           Methylenedianiline     II     447.1                                           Trimellitic anhydride  II     861.0                                           ______________________________________                                    

The contents are slowly heated with the evolution of water to a maximumtemperature 208° C. until a clear resin of acid number 1.1 is obtained.The batch is cooled to 145°-150° C. and Part II is added. The heating isresumed to a maximum temperature of 213° C. and an acid number of lessthan 7 is obtained. The batch is cooled to 150° C. and 240.55 g of 40%solution of an alkanol-blocked polyisocyanate (Mobay KL5-7005) indiethylene glycol monomethyl ether is added. The resin is stirred for anadditional 30 minutes and the hot resin is poured directly into cans. Itis a solid at room temperature and has a solids content of 93.9%. It isapplied from the melt at 170° C. to copper wire to produce a coatedconductor.

The enamel is cured in a 15 foot vertical tower at 700° F. on 18 AWGcopper magnet wire in 4 passes at 34 ft./min. to produce a coated wire.Properties are measured in conventional ways, and the results are setforth as follows:

    ______________________________________                                        Wire Properties                                                               ______________________________________                                        Dissipation Factor at 220° C.                                                                 5.2                                                    Cut Thru °C.    396                                                    Diel. Strength, kv     8.2                                                    Heat Age, 21 hrs. at 175° C., 0% +                                                            3X                                                     Repeat Scrape           57                                                    ______________________________________                                    

The flexibility retention of the enamels produced according to thisinvention is outstanding.

Dual coated wires are made in a tower as described above.

In the first, a base coat of a polyester of dimethyl terephthalate,ethylene glycol and glyerine made according to Precopio et al., U.S.Pat. No. 2,936,296 is applied to a build of about 2.3 mls. To thiscoating is then applied a thinner, 0.3 mil. over coating of thepolyesterimide of the Example. A coated copper conductor according tothis invention is obtained.

In the second, a wire coated with the polyesterimide of this invention(Example 1) has applied to it a thin outer coating of an amide-imidemade by mixing and heating trimellitic anhydride and the diisocyanate ofmethylene dianiline. A coated copper conductor according to thisinvention is obtained.

It is obviously possible to make many variations in the presentinvention in light of the above, detailed description. For example, thealkyl titanate can be omitted. Phenol-formaldehyde resin can be added orit can be substituted with a melamine-formaldehyde resin. Metal drierscan also be added, e.g., 0.2 to 1.0% based on total solids, of zincoctoate, cadmium linoleate, calcium octoate, and the like. Instead ofthe monomethyl ether of diethylene glycol used as co-reactant, there canbe used the monobutyl ether of diethylene glycol. The followingmonoester alcohols can be used: ethylene glycol monoacetate anddiethylene glycol monoacetate. All such obvious variations are withinthe full intended scope of the appended claims.

We claim:
 1. An electrical conductor provided with a coating of acomposition comprising a resinous reaction product obtained by heatingingredients comprising:(a) an aromatic diamine; (b) an aromaticcarboxylic anhydride containing at least one additional carboxylicgroup; (c) terephthalic acid or a reactive derivative thereof; (d) apolyhydric alcohol having at least three hydroxyl groups; (e) analkylene glycol; and (f) a small amount of(i) an alkylene glycolmonoether or monoester; (ii) a polyalkylene glycol monoether ormonoester; or (iii) a mixture of (i) and (ii),until an acid number oflower than about 6 to about 7 is attained, and adding to the hotreaction product a blocked polyisocyanate in an amount of from about 1to about 40 parts by weight per 100 parts by weight of thepolyesterimide and blocked polyisocyanate.
 2. An electrical conductor ofclaim 1, and a lesser wall of a second coating of a wire enamel of adifferent type covering said reaction product coating.
 3. An electricalconductor of claim 1, provided with a first coating of a wire enamelwhich is free of said reaction product and a lesser wall of reactionproduct covering said first coatings of wire enamel.
 4. An electricalconductor provided with a coating of a composition comprising a resinousreaction product obtained by heating at from about 180° C. to about 220°C. the ingredients comprising:(a) methylene dianiline; (b) trimelliticanhydride; (c) terephthalic acid; (d) tris(2-hydroxyethyl)isocyanate;(e) ethylene glycol; and (f) diethylene glycol monomethyl ether,until anacid number of lower than 6 to 7 is attained, and adding to the hotreaction product at a temperature of from about 120° to about 180° C. ablocked polyisocyanate comprising mixed cyclic trimers of 2,4- and2,6-tolylene diisocyanates having the three free isocyanate groupsblocked by esterification with m-cresol or an alkanol of from 1 to 6carbon atoms, the amount of blocked polyisocyanate being from 1 to 15parts by weight per 100 parts by weight of the polyesterimide and theblocked polyisocyanate.
 5. An electrical conductor of claim 4, and alesser wall of second coating of a wire enamel of a different typecovering said reaction product coating.
 6. An electrical conductor ofclaim 4, provided first with a coating of a wire enamel which is free ofsaid reaction product and a lesser wall of reaction product coveringsaid first wire enamel.